(A) General Design Criteria for Limestone Contactors Installed in Small Water Systems
A designer of limestone contactor should be aware of the various factors affecting limestone dissolution before designing a contactor to ensure its optimal
performance. In summary, the factors that can affect the dissolution of limestone include the quality of water to be stabilized and the physical and chemical
characteristics of limestone to be used.
Design Criteria
Location in the Treatment
Train
U.S.
Limestone contactors are typically
placed after filtration, primary
disinfection and chlorine contact.
(Spencer, 2000).
Germany
South Africa
Limestone contactors should be placed after
disinfection (Stauder, 2002).
Limestone contactors should be
placed after clarification,
filtration and chlorination
(Mackintosh, De Souza and De
Villiers, 2003a).
In order to determine whether a limestone
contactor is feasible to treat a water system,
the following water quality parameters must be
known (DVGW, 1998):
The iron levels must be < 0.1
mg/L, aluminum levels < 0.15
mg/L and turbidity < 1 NTU
(Mackintosh, De Souza and De
Villiers, 2003a).
There was also a pilot study conducted
in British Columbia (Benjamin et
al.,1992) that used a limestone contactor
before flocculation to condition the
water.
Note: Besides the typical locations noted
here, there have been suggestions that
limestone contactors could also be used
as a pretreatment unit to adjust pH for
removal of iron, manganese and
aluminum.
Water Quality
Characteristics
The water to be treated with a limestone
contactor must have low iron and
turbidity content (Snoeyink et al., 1997).
The USEPA (2003) prepared a flow
diagram (Figure 1) to determine whether
a limestone contactor would be a
feasible option for a water system. The
decision would be based on assessing
the water quality parameters of the
system.
In summary, a water system should have
a pH < 7.2, calcium < 60 mg/L and
alkalinity < 100 mg/L. A special
contactor design is required if the iron
• Acid consumption, KS,4.3
The acid consumption, KS,4.3 is the acid
consumption in meq/liter to achieve a pH
of 4.3. This value multiplied by 61
approximately equals the concentration of
hydrogen carbonate, HCO3- in ppm
(Beforth, 2003).
• Base consumption, KB,8.2
The base consumption, KB,8.2 is the amount
of base consumed by water in meq/liter to
content is more than 0.2 mg/L and
manganese content is more than 0.05
mg/L. This is due to possibility of the
iron and manganese coating the
contactor, and slows the calcium
carbonate dissolution (USEPA, 2003).
The Spraystab I design has been used in
South Africa for treatment of water that
contains more than 0.2 mg/L of iron and
0.05 mg/L or manganese and treated
with a limestone contactor. The water
can first be aerated, then stabilized with
limestone contactor and finally filtered
through a sand filter.
The values in Figure 1 are similar to
those given by Spencer (2000) except
that alkalinity was given as < 50 mg/L
and hardness < 50 mg/L.
In addition to the above, the designer
must know the temperature range, total
inorganic carbon levels (DIC) and total
dissolved solids or conductivity.
achieve a pH of 8.2 (Beforth, 2003). This
value multiplied by 44 approximately
equals the concentration of carbon dioxide,
CO2 in ppm.
• Calcium concentration
• Saturation pH value, pHc after addition of
calcite.
According to Beforth (2003), since the
reactivity of limestone is very low, one would
find that KS,4.3 and KB,8.2 should be very low to
achieve a pH in a range of 7.7 – 8.0.
In addition, iron, manganese and aluminum
levels must be less than the following
concentrations to prevent formation of residue
on the surface of the limestone media
(DVGW, 1998):
• Iron < 0.2 mg/L
• Manganese < 0.05 mg/L
• Aluminum < 0.05 mg/L
If residues are formed, then iron, manganese,
or aluminum removal may be required ahead
of the limestone contactor (DVGW, 1998).
Empty Bed Contact Time
(EBCT)
EBCT is in the range of 20 to 40
minutes if water is less than 5 oC,
otherwise, EBCT in the range of 15
minutes to 1 hour have been used in
successful contactor design. (Spencer,
2000).
EBCT is the most important design criteria in
Germany. EBCT usually varies between 20
and 45 minutes (0.33-0.75 h), depending on
the CO2 and HCO3- contents in the raw water
(Stauder, 2002).
EBCT more than 20 minutes
unless determined to be
otherwise on-site. (Mackintosh,
De Souza and De Villiers,
2003a).
Loading Rate
2.4 m/hr or 1 gpm/ft2 (Spencer, 2000).
The typical loading rates are in the range of 4 8 m/hr or 1.7 - 3.3 gpm/ft2 for common bed
depth of 2 to 3 m. (Stauder, 2003).
Less than 10 m/hr or 4.1
gpm/ft2 (Mackintosh, De Souza
and De Villiers, 2003a).
Ensure equal flow into all
limestone contactors, and the
flow should be less than the
maximum design capacity.
(Mackintosh, De Souza and De
Villiers, 2003a).
Flow Requirements
Limestone contactors must be designed to The flow to the limestone contactor must not
treat the maximum flow of the plant.
exceed the maximum design capacity
(Spencer, 2000).
(DVGW, 1998). If dolomite is used as a
media, the flow rate should not fall below the
design flow rate by more than 30% (DVGW,
1998) to avoid the water from becoming
oversaturated with respect to CaCO3 and
precipitating in the contactor (Stauder, 2003).
Limestone Bed Depth
Limestone bed depth can be determined
by calculation using computer programs
such as DESCON and Limestone Bed
Contactor: Corrosion Control and
Treatment Analysis Program Version
1.02. (Please refer to the design aids).
The limestone bed depth can be determined
using the appropriate Figure 4 to 8 and the
equations discussed in the design aids section.
However, the typical limestone bed depth is in
the range of 2 to 3 m (Stauder, 2003). If
dolomite is used as a media, the total filter bed
volume should be distributed across several
filters to minimize pH fluctutations.
Minimum limestone bed depth
of 2 m is recommended
(Mackintosh, De Souza and De
Villiers, 2003a).
Limestone Characteristics
Limestone used should be that described
in ASTM Standard C51-02 as
“sedimentary rock consisting of mainly
carbonate and containing no more than 0
to 5% magnesium carbonate” (Spencer,
2000). This standard is available from
http://www.astm.org.
There are several types of limestone media
discussed by DVGW(1998) that can be used in
a limestone contactor. These include dense
calcium carbonate, porous calcium carbonate
and dolomite. Dolomite is suitable to be used
in a large contactor since it will provide a
lower contact time (Stauder, 2002) although it
also has some disadvantages (Please refer to
the flow requirements section). The following
shows the water quality characteristics that are
applicable for each type of the limestone
media (DVGW, 1998):
Limestone used should consists
of high calcium and low
magnesium stone. Currently,
limestone used to date in
several installations in South
Africa is the commercially
available limestone pebbles
from Bredasdorp, South
Western Cape (De Souza et.
al., 2000 and Mackintosh et.
al., 2003a). It consists of 96%
calcium, 1.7% silica, 1.3%
magnesium and less than 0.1%
iron and manganese by mass.
The weighted sum of aluminum and iron
in stone must not exceed 10 mg/g of
stone (Letterman, 1995).
Testing of constituents of stone must
adhere to the ASTM Standard 25-99
(Chemical Analysis of Limestone,
Quicklime and Hydrated Lime)
(Spencer, 2000).
The USEPA (2003) has expressed
concern that the limestone should not
contain significant amounts of materials
such as heavy metals that would be
a) Dense Calcium Carbonate
i. Target pH=8
KS,4.3 + 2KB,8.2 < 1.5 mol/m3
[Ca] < 0.75 mol/m3
ii. Target pH=saturation pH
KS,4.3 + 2KB,8.2 < 1.0 mol/m3
[Ca] < 0.75 mol/m3
b) Porous Calcium Carbonate
KS,4.3 + 2KB,8.2 < 1.5 mol/m3
imparted to the water as the limestone
dissolves.
[Ca] < 0.75 mol/m3
c) Half-Burnt Dolomite
KS,4.3 + 2KB,8.2 < 2.5 mol/m3
Depending on the type of limestone used
(either calcium carbonate or half-burnt
dolomite), it must comply with the
requirements outlined in the following British
Standards:
• BS EN 1018:1998 (Chemicals used for
treatment of water intended for human
consumption – Calcium Carbonate).
• BS EN 1017:1998 (Chemicals used for
treatment of water intended for human
consumption – Half Burnt Dolomite).
These documents are available from
http://www.techstreet.com/info/bsi.html
Limestone Grain Sizes
Repeated runs using DESCON revealed
that 11 to 14 mm stone (0.4 to 0.55 inch)
seems to be an optimal size. (Spencer,
2000).
The following grain sizes are commercially
available for calcium carbonate and half-burnt
dolomite.
• Dense Calcium Carbonate : 0.71-1.25 mm,
1.0-2.0 mm, 1.6-2.5 mm
• Porous Calcium Carbonate : 1.0 – 3.0 mm
• Half-Burnt Dolomite: 0.5-1.2 mm, 0.5-2.5
mm, 2.0-4.5 mm, 4.0-7.0mm
( Note: If undersized or oversize grains are
present, they cannot be larger than 10% for
the individual group.)
However, size range of 1.0-2.0 mm is
commonly used in Germany. Although, in
some of the older large water treatment plants
that are not able to properly backwash, larger
media sizes are used. Based on experience,
there is not a significant change in the
Limestone used has a grading
of +12mm – 15mm (De Souza
et. al., 2000). The size of
limestone that produced
optimal results is 12 mm
(Spencer, 2000).
reactivity of the media for the size range
between 0.7-3.0 mm (Stauder, 2003).
Supporting Media
Supporting media used is based on the
AWWA Standards for Filtering and
Support Media (Spencer, 2002). The
size of the support media used should be
larger than the limestone size (Spencer,
2002).
Supporting media consists of 0.2 – 0.3 m in
depth of inert materials (DVGW, 1998).
Supporting media consists of
150 mm deep, 25 mm diameter
granite aggregrate
(Mackintosh, De Souza and De
Villiers, 2003a).
Design Aids
DESCON model was developed by
Letterman and Kothari (1995) to aid in
the design of limestone contactors. This
computer program has been used by
Spencer (2002) to design the limestone
contactor facility in Mars Hill, Maine.
This program is also recommended for
the design of a limestone contactor by
USEPA (2003).
A guideline for designing a limestone
contactor is available (W 214/II: Entsaurung
von Wasser Teil 2: Grundsatze fur Planung,
Betrieb und Unterhaltung von Filteranlagen)
from DVGW (1998). However, this guideline
is based on empirical correlations. Therefore,
Stauder (2003) suggests that for smaller plants,
the limestone bed volume can be designed
using the guideline and for larger plants, the
limestone bed volume should be designed
based on a pilot plant study. Stauder (2003)
also suggests that it is very important (before
designing) to obtain a reliable water analysis
since any erroneous values can greatly affect
the design. Computer software that can
determine the calcium carbonate saturation of
water is also very useful.
CaCO3 saturation
characteristics of water can be
determined experimentally
using the Marble Test. The
Marble Test is an experiment
based method to determine
calcium carbonate saturation
characteristics of water.
DESCON calculates the depth of
limestone required in a contactor based
on the influent water chemistry,
limestone particle size, superficial
velocity and the desired effluent water
chemistry (Letterman, 1995).
Figure 2 shows an output from
DESCON using USEPA (2003) data
from a typical Western Water System as
input to the program. In this system,
surface water contributes 80% of the
amount of water supplied.
The limestone diameter was entered as
12 mm and limestone CaCO3 content as
98%. DESCON calculated the
equilibrium pH to be 8.82 and it
required user input for the target pH.
Figure 4 to 8 are used to determine contact
time using base and acid consumption of water
at temperature of 10 oC for each of the
following conditions. The contact time is then
used to determine the minimum volume of
media required in a contactor for several
alternatives:
(i) Dense calcium carbonate with grain size
ranges between 1.0 – 2.0 mm and a target
pH value of either 8 or pHc.
Computer program such as
STASOFT also used to
determine the CaCO3 saturation
and degree of stabilization of
water using CaCO3.
The target pH must be less than the
equilibrium pH. Therefore, the target pH
was set at 8.5.
Based on the DESCON output, the depth
of the limestone contactor to reach a
target pH of 8.5 for this system was
calculated as 3.1 ft.
Another program used to design a
limestone contactor was developed by
Schott (2002) called Limestone Bed
Contactor: Corrosion Control and
Treatment Process Analysis Program
Version 1.02. This program calculates
the depth of the limestone bed, empty
bed contact time and the limestone
dissolved per volume of water treated
from the initial to equilibrium condition.
Other additional parameters that are
calculated using this program include
pH, total alkalinity, CO2 concentration,
DIC, calcium content and copper
content from the initial to equilibrium
condition. Figure 3 shows example of
the software output.
(ii) Porous calcium carbonate with grain size
ranges between 1.0 – 3.0 mm and a target
pH value of either 8 or pHc.
(iii)Half-burnt dolomite with grain size ranges
between 0.5 – 2.5 mm and a target pH
value of pHc.
If the grain sizes used are not within the range
specified in the figures, manufacturer’s
recommendations should be used in designing
a contactor.
Figure 9 is used for temperature correction.
Using Figure 4 to 8, the minimum volume of
the limestone media required can be calculated
as follows (DVGW, 1998):
(i) Minimum volume of Limestone,VM:
VM = Q x tf x f x 1/60
where:
VM = minimum volume of media, m3 chosen
using the appropriate Figure 4 to 8 (For
dense CaCO3, VM may be lower than
calculated if the target pH is less than
8.0 or pHc (Stauder, 2003). Currently in
Germany, the enforceable pH for
corrosion control is around 7.4 to 7.6.
Therefore, the VM used can be about
half of the calculated.)
Q = flow rate, m3/hr
tf = hypothetical contact time, min
f = temperature correction factor
(refer to Figure 9)
Additional media, VB, is then added to the VM
to account for media consumption during the
period between two media fillings and
backwashing (Stauder, 2003). Since VM is the
minimum amount of media required in a
contactor to achieve the desired pH, therefore
VB must be added so that the volume of the
media after consumption will never be less
than VM.
According to Stauder (2003), it requires
experience to determine VB. It may depend on
several factors besides media consumed due to
the acidity of water or backwashing, such as
the capacity of the truck used to transport the
media, the size of the silo and the distance
between the water treatment facility and the
limestone supplier facility. However, VB can
be estimated based on the following (Stauder,
2003):
Estimation of VB:
The amount of media consumed can be
calculated based on the acidity of the water or
the amount of CO2 present in the water (can be
calculated from KB8.2). For example, for every
mol of CO2 reacting with CaCO3 in water, 100
g of CaCO3 is consumed according to the
following reaction:
CO2 + CaCO3 + H2O = Ca2+ + 2HCO3If dolomite is used, for every mol of CO2
reacting with dolomite in water, 47 g of
dolomite is consumed as determined
empirically. Using the densities of CaCO3 and
dolomite, the volume of media consumed can
be calculated. In addition, 10% can be added
to account for media losses during
backwashing.
The sum of VB and VM is the volume of the
limestone bed, VF calculated as follows
(DVGW, 1998):
(ii) Filter Bed Volume, VF:
VF = VM + VB
where:
VF = filter bed volume, m3
VB = consumption volume, m3
Click on this link to view an example on how
to use Figure 4 to 10.
Influent Distribution
Systems
Piping Requirements
A bypass around the contactor and a
drain within the contactor must be
available to allow annual maintenance of
a contactor (Spencer, 2000).
Installation Requirements
Contactor Configuration
Both cylindrical and box-shaped
contactors have been designed in the
U.S.
For small contactor facilities, there is no
requirement on the influent distribution system
as long as the contactors can be filled with
water (Stauder, 2003). Special distribution
system is only needed for large contactors.
Use a false bottom feed
systems instead of
manifold/slotted pipe or other
type systems (Mackintosh, De
Souza and De Villiers, 2003a).
Limestone contactors need to be equipped with
devices for drainage of initial filtrate (DVGW,
1998).
Piping to and from the units
should be such that each
individual contactor is able to
operate independently, be
filled independently, be
flushed to waste in both
upflow and downflow mode
and handle excessive flow
loading via an overflow pipe to
waste (Mackintosh, De Souza
and De Villiers, 2003a).
The number of contactors that need to be
installed depends on the site specification and
plant throughput (Stauder, 2003).
At least two contactors should
be installed for each water
system to allow uninterrupted
operation while one of the
contactors is under
maintenance (Mackintosh, De
Souza and De Villiers, 2003a).
Most of the older water systems are
rectangular open tanks and most of the newer
water systems are cylindrical pressure tanks
(Stauder, 2003).
Cylindrical configuration, with
ratio of height to diameter of at
least 1:1. The structure should
be completely enclosed, with
access hatch on top for
limestone addition.
(Mackintosh, De Souza and De
Villiers, 2003a).
However, the configuration depends on the
amount of water to treat and the benefit of
keeping the pressure (Stauder, 2003).
Rectangular configuration is
also possible. However, the
hydraulics of the system must
be taken into consideration.
The ratio of height to wall
length must be at least 1:1 and
the corners of the contactor
must be benched/curved. The
curved benching must be at
least one quarter of the wall
length.
Construction Material
Concrete is usually used for open contactors
and stainless steel is typically used for closed
contactors (Stauder, 2003).
Cement-concrete or fiberglass
(Mackintosh, De Souza and De
Villiers, 2003a).
Contactor Wall Protection
For open contactors, the wall can be protected
from corrosion using ceramic tiles while in the
closed contactors, the wall can be protected
with epoxy coating (Stauder, 2003).
The internal contactor wall
should be coated (for example
with epoxy coating) to protect
it from aggressive or abrasive
reaction. (Mackintosh, De
Souza and De Villiers, 2003a).
Backwash Requirements
Depending on the type of media, the limestone
contactors should be backwashed at least once
a week (DVGW, 1998). If the head loss
increases above the allowed value,
backwashing needs to be done more
frequently.
The limestone contactors must
be able to down-flush fines to
waste at least once a month
until the site-specific frequency
is determined (Mackintosh, De
Souza and De Villers, 2003a).
The following shows a successful backwash
practice in Germany (DVGW, 1998):
Backwash
Duration Backwash Rate
(m/hr)
Step
(minutes)
Air
Water
1 Air
3–5
60
None
10 – 12
2 Air/Water 5 – 10
60
3 Water
None
None 12 – 25
a
Specified
a
The common duration is 10 – 15 minutes
(Stauder, 2003).
According to DVGW (1998), the dirty water
from the backwash process contains solids
(such as iron flocs, manganese flocs,
aluminum flocs and undersized media) that
may be basic. This is especially true for
contactors using half-calcine dolomitic lime
and if backwashing is carried out for the first
time on a new media or after long idle periods.
The backwash water does require proper
disposal.
Water Quality Monitoring
Requirements
Limestone contactors must be equipped with
water quality monitoring taps before and after
stabilization (DVGW, 1998). The most
important parameter to measure is pH of the
effluent from the contactor especially for a
contactor using dolomitic limestone as a
media.
Limestone contactors should have visible
indicator showing the level for media refilling
Provide sample taps for water
quality monitoring before and
after stabilization.
(Mackintosh, De Souza and De
Villiers, 2003a).
Provide two piezometers on
each contactor to measure
pressure loss across the
limestone bed (Mackintosh, De
(DVGW, 1998). The media refill level is
determined from the filter bed volume and
minimum volume.
Media Refilling Facility
Limestone contactors must be equipped with
devices for measuring head loss (DVGW,
1998).
The media can be refilled into the contactor
either manually or hydraulically, but
hydraulically is preferred (DVGW, 1998).
Souza and De Villiers, 2003a).
Pressure loss measurement is
necessary to indicate that the
limestone fines in the contactor
need to be “down flushed”.
The media can be refilled in a
contactor manually or using a
loading gantry facility
(Mackintosh, De Souza and De
Villiers, 2003). Usually, media
refills are done manually if the
limestone comes in lighter
packages (i.e. 25 kg bags), but
typically in the recent large
units, a loading gantry facility
is used since the limestone
comes in heavier packages (1
ton bags).
FIGURES
Figure 1. Limestone Contactor Decision Tree (Spencer, 2003)
Figure 2. DESCON Output Using the Typical Western Water System Data as Input
Figure 3. Limestone Bed Contactor Corrosion Control and Treatment Analysis Program Version 1.02 (Schott, 2003)
Figure 4. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Dense Calcium Carbonate (Grain Size 1.02.0 mm). Temperature: 10oC, Processing Goal pH=8.0. (DVGW, 1998)
Figure 5. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Dense Calcium Carbonate (Grain Size 1.02.0 mm). Temperature: 10oC, Processing Goal pH=pHc. (DVGW, 1998)
Figure 6. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Porous Calcium Carbonate (Grain Size 1.03.0 mm). Temperature: 10oC, Processing Goal pH=8.0. (DVGW, 1998)
Figure 7. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Porous Calcium Carbonate (Grain Size 1.03.0 mm). Temperature: 10oC, Processing Goal pH=pHc. (DVGW, 1998)
Figure 8. Contact Time as a function of Base Capacity (KB,8.2) and Acid Capacity (KS,4.3) for Half-Calcine Dolomitic Lime (Grain Size 0.5-2.5 mm).
Temperature: 10oC, Processing Goal pH=pHc. (DVGW, 1998)
Figure 9. Correction Factor as a Function of Water Temperature. (DVGW, 1998)
SAMPLE PROBLEM
Sample on how to use Figure 1 to 6 in W214/11 (DVGW, 1998):
This example shows how to design a limestone contactor by using figure 1 to 6 in W214/11 (DVGW, 1998). Limestone contactor will be used to treat the
following raw water:
Raw Water* Treated by Mars Hill, Maine Water Treatment Plant (Spencer, 2000):
Surface Water Source: Young’s Lake
Lowest pH of influent to the limestone contactor = 7.18
Alkalinity in the raw water = 45 – 83 mg/L as CaCO3
Calcium in the raw water = 19 – 34 mg Ca/L
*Note: These water quality parameters were obtained in 1996, before the limestone contactor was installed in the plant.
Determination of KS,4.3 and KB,8.2:
Use the worst case condition, pH = 7.18 and alkalinity of 45 mg/L as CaCO3
Most of the alkalinity source at pH = 7.18 is from HCO3-:
KS,4.3 = [HCO3-] = 45 mg x 1 meq x 1 mmol HCO3- x
1 mol x 103 L = 0.9 mol
3
L
50mg 1 meq HCO3
10 mmol 1 m3
m3
Assume the lowest temperature, T = 5 oC,
Therefore pKa1 = 6.52 at T = 5 oC (Snoeyink and Jenkins, 1980). Assume CO2 = H2CO3* and ignoring activity coefficients:
KB,8.2 = [CO2] = [H2CO3*]
[HCO3-][H+] = 10-pKa1
[H2CO3*]
KB,8.2 = [CO2] = [H2CO3*] = (0.9 x 10-3)(10-7.18) mol x 103 L = 0.2 mol
( 10 -6.52 )
L
m3
m3
Determination of Media Type:
KS,4.3 + 2KB,8.2 = 0.9 + 2(0.2) = 1.3 mol/m3
[Ca] = 19 mg x mmol x
mol
x 103 L = 0.47 mol/m3
3
L
40.08 mg 10 mmol
m3
Based on the total amount of KS,4.3 + 2KB,8.2, the calcium content and the requirement listed in Part 3.2 of W 214/11 (1998), the following media can be
used:
• Dense Calcium Carbonate, Target Goal pH=8. Figure 1 of W214/11 is applicable.
• Porous Calcium Carbonate, Target Goal pH=8 or pH=pHc. Figure 3 of W214/11 is applicable.
• Dolomite, Target Goal pH=pHc. Figure 5 of W214/11 is applicable.
Applicability of Design Figures:
Figure 1, 3 and 5 of W214/11 (or Figure 4,6 and 8 in this module) can be used to determine the contact time based on KS,4.3 and KB,8.2. Figure 6 of
W214/11 (or Figure 9 in this module) is used for temperature correction.
Design Method:
Maximum flow rate, Q = 1600 m3/day (430,000 gallons/day) (Spencer, 2000)
Loading Rate = 2.4 m/hr (1 gpm/ft2) (Spencer, 2000)
Assume lowest temperature, T = 5 oC.
(i)
If using Dense Calcium Carbonate and the desired target pH=8:
Assume size of media = 1.0 – 2.0 mm
Target pH = 8
From Figure 1, tF = 25 min and from Figure 6, f = 1.5.
Minimum bed volume, VM
= tF x f x Q x 1/60 min
= 25 min x 1.5 x 1600 m3/day x 1 day/24hr x 1 hr/60 min
= 42 m3
Depth of limestone bed = tF x f x Loading Rate
= 25 min x 1.5 x 2.4 m/hr x 1 hr/60 min
= 1.5 m (5 ft)
Surface area of the contactor = 1600 m3/day x 2.4 m/hr x 1 day/24 hr = 28 m2
Figure 1. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Dense Calcium Carbonate (Grain
Size 1.0-2.0 mm). Temperature: 10oC, Processing Goal pH=8.0. (DVGW, 1998)
tF = 25 min
Figure 6. Correction Factor as a Function of Water Temperature. (DVGW, 1998)
f = 1.5
Determination Whether VB is Needed in the Design
If the dissolution rate of the limestone is very slow, then a consumption volume, VB is not needed (Stauder, 2003). The need to include VB in the
design of a limestone contactor is up to the designer. The choice depends much on the frequency of media refilling. Additional factors include the
capacity of the limestone supplier truck and the capacity of the silo used to store the limestone (Stauder, 2003). If the capacity of the truck and the
silo is small, it may limit the amount of limestone that can be refilled each time, therefore, the refill frequency must be chosen by taking these
factors into account. The amount of CaCO3 dissolved for several options of media refills frequency is shown in this example.
A run using MINTEQ reveals that at saturation, this water dissolves 0.008 g/L of Ca2+ (0.1988 mol/m3). The following calculations show the
amount of CaCO3 dissolved daily, weekly, monthly and yearly based on the result obtained using MINTEQ.
The daily consumption of CaCO3
= 1600 m3/day x 0.1988 mol/m3 x 100 g/mol x 10-3 kg/g
= 32 kg/day
Weekly consumption of CaCO3 = 31.8 kg/day x 7 days/week
= 223 kg/week
Monthly consumption of CaCO3 = 31.8 kg/day x 30 days/month = 954 kg/month
Yearly consumption of CaCO3 = 31.8 kg/day x 365 days/yr = 11,607 kg/year
Using the density of dense CaCO3 of 1500 kg/m³ (Table 1 in W214, DVGW 1998), the weight of CaCO3 can be calculated. Based on Figure 1 and
6, the minimum volume, VM of limestone needed to stabilize this water is 42 m³. This corresponds to 63,000 kg of CaCO3. The following table
shows the percent of CaCO3 dissolved daily, weekly, monthly and yearly.
No.
1
2
3
4
Frequency of Media Refilling
Daily
Weekly
Monthly
Yearly
Amount of CaCO3
Dissolved
32 kg/day
223 kg/week
954 kg/month
11,607 kg/year
Percent of CaCO3
Dissolved (%)
0.05
0.35
1.51
18.42
The above table shows that if the designer decides to refill the limestone either daily or weekly, the percent of CaCO3 dissolved between media
refilling is very small (less than 1%). Since the amount dissolved is very small, the designer may exclude VB in the design if he/she decides to
have the limestone refilled daily or weekly. Thus, the total contactor bed volume should only include the minimum volume of limestone required,
VM. However, if a designer decides to have the limestone refilled monthly or yearly, then VB must be included in the design since a significant
amount of limestone will be dissolved between media refilling.
The amount of VB can be calculated based on the amount of CaCO3 dissolved as calculated above (954 kg for monthly refill and 11,607 kg for
yearly refills) with the addition of 10% to account for losses of media due to backwashing. The following shows an example to calculate VB, the
total contactor bed volume, VF and the total height of contactor for monthly media refills.
VB = 954 kg / 1500 kg/m3 + 0.10 ( 42 m3 + 954 / 1500 kg/m3 ) = 5 m3
VF = 42 m3 + 5 m3 = 47 m3
Total Height of Contactor = 47 m3 / 28 m2 = 1.7 m (5.6 ft)
(ii)
If using Porous Calcium Carbonate and the desired target pH = 8:
Assume size of media = 1.0 – 3.0 mm
Target pH = 8
From Figure 3, tF = 11.5 min and from Figure 6, f = 1.5.
Minimum bed volume, VM
= tF x f x Q x 1/60 min
= 11.5 min x 1.5 x 1600 m3/day x 1 day/24hr x 1 hr/60 min
= 19.2 m3
Depth of limestone bed = tF x f x Loading Rate
= 11.5 min x 1.5 x 2.4 m/hr x 1 hr/60 min
= 0.7 m (2.2 ft)
Surface area of the contactor = 1600 m3/day x 2.4 m/hr x 1 day/24 hr = 28 m2
Figure 3. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Porous Calcium Carbonate (Grain
Size 1.0-3.0 mm). Temperature: 10oC, Processing Goal pH=8.0. (DVGW, 1998)
tF = 11.5 min
(iii)
If using Dolomite and the desired target pH = pHc
Assume size of media = 0.5 – 2.5 mm
Target pH = saturation pH = pHc
From Figure 5, tF = 7.8 min and from Figure 6, f = 1.5.
Minimum bed volume, VM
= tF x f x Q x 1/60 min
= 6.8 min x 1.5 x 1600 m3/day x 1 day/24hr x 1 hr/60 min
= 11 m3
Depth of limestone bed = tF x f x Loading Rate
= 6.8 min x 1.5 x 2.4 m/hr x 1 hr/60 min
= 0.4 m (1.5 ft)
Surface area of the contactor = 1600 m3/day x 2.4 m/hr x 1 day/24 hr = 28 m2
Figure 5. Contact Time as a function of Base Consumption (KB,8.2) and Acid Consumption (KS,4.3) for Porous Calcium Carbonate (Grain
Size 1.0-3.0 mm). Temperature: 10oC, Processing Goal pH=pHc. (DVGW, 1998)
tF = 6.8 min
(B) Design Criteria Specifically for Small Groundwater Systems with Iron and Manganese Present
Table 1 shows the treatment objectives and Table 2 shows the installation and operation requirements applicable for the Spraystab I unit used to stabilize
water in small-scale groundwater systems containing iron and manganese in South Africa.
Table 1. Treatment Objectives of Spraystab I Unit (Mackintosh, Engel and De Villiers 1997)
No.
Treatment Objectives
1. The unit should achieve an appreciable level of pH adjustment and stabilization.
2. The unit should be able to remove iron such that the iron level in the treated water is no
more than 1 mg/L and preferably less than 0.3 mg/L.
3. The unit should be able to remove manganese such that the manganese level in the
treated water is no more than 1 mg/L and preferably less than 0.1 mg/L.
4. The unit should be able to filter the water.
5. The unit should be able to treat between 25 and 50 m3/day.
Table 2. Installation and Operation Requirements of Spraystab I Unit (Mackintosh, Engel and De Villiers, 1997)
No.
Installation and Operation Requirements
1. All components of the unit are easily handled by two people and easily transported.
2. The materials and equipment needed should result in a low cost and easy construction and
repair.
3. The unit requires minimum operator attention and skills.
4. The unit utilizes minimum chemical dosing. If dosing is required, it should be selfregulating.
5. The unit should not require dosing pumps.
6. The unit should not require water pumps other than the well pump.
7. The unit must be robust and reliable.
8. The unit is independent of electrical control or operating systems.
Table 3 shows the design criteria of the Spraystab I unit.
Table 3. General Design Criteria of Spraystab I System (Mackintosh, De Souza and De Villiers, 2003b)
No.
1.
Components
Access Lid
2.
Spray Nozzles
3.
Aeration Ducts
4.
Vent Ducts
5.
Limestone Media
6.
Support Grid
7.
Slotted Pipe
Underdrain
8.
Filtration Media
Functions
An access lid must be provided on a closed contactor to allow
access for maintenance or cleaning work, prevent contamination
from outside materials such as leaves and twigs, prevent the beds
from algal/bacterial growth and minimize dissolution of carbon
dioxide into the water.
The 60o full cone spray nozzles are located at the center of the
aeration ducts. The well pumps feeds directly to the spray nozzles.
The nozzle height above the aeration duct is designed such that the
air flowing into the aeration duct is at the maximum.
The aeration ducts are located on the access lid and covered with
coarse stainless steel wire mesh screens.
The vent ducts are covered with coarse stainless steel wire mesh
screens.
The limestone contact unit consists of a bed of approximately 800
mm in depth of limestone with a grading of –15mm +12mm. The
bed rests on top of the support grid.
The support grid is located at the lower end of the limestone
contactor unit of a Spraystab I unit. There is a 10 mm wire mesh
screen placed on the support grid.
Each compartment in the filtration unit of a Spraystab I unit is
equipped with a slotted pipe underdrain. The slotted pipe feeds into
a common manifold fitted with a raised outlet used to control the
water level in the filtration unit.
There are two media used in the filtration unit. The lower layer of
the filtration unit consists of 0.3 to 0.5 mm graded filter sand to a
depth of 300 mm above the underdrain. The upper layer of the
filtration unit consists of 1.0 to 1.5 mm graded hydro-anthracite
media.
(C) General Design Criteria Specifically for Small Groundwater Systems without Iron and Manganese Present
Table 4 shows the design criteria of the Spraystab II system used to stabilize water in small-scale groundwater systems without iron and manganese
present.
Table 4. General Design Criteria of Spraystab II System (Mackintosh, De Souza and De Villers, 2003b)
No.
Components
1. Access Lid
2.
Spray Nozzles
3.
Aeration Ducts
4.
Vent Ducts
5.
“Settling Tubes”
6.
7.
Slotted Pipe
Granite Layer
8.
9.
Drain/Flush Valves
Treated Water
10.
Treated Water
Outlet Pipe
Loading Rate
11.
Functions
An access lid must be provided on a closed contactor to allow access for maintenance or cleaning work, prevent
contamination from outside materials such as leaves and twigs, prevent the beds from algal/bacterial growth and
minimize dissolution of carbon dioxide into the water.
The type of material used for spray nozzles is important. The full cone HH stainless steel spray nozzles are more effective
in aerating water and durable compared to PVC/plastic spray nozzles. In addition, PVC/plastic spray nozzles are also
more prone to clogging resulting in poor performance in aeration.
Each limestone contactor unit is required to have two aeration ducts (i.e. air inlets). Each duct have a diameter of 250
mm. The ducts must be covered with a stainless steel mesh to avoid leaves, twigs and other outside materials from
building up on the surface of the limestone beds.
Each limestone contactor unit is required to have two vent ducts, one on the access lid and the other on the roof of the
tank. Each vent duct should have a diameter of 300 mm. Similar to the aeration ducts, the vent ducts mush also be
covered with a stainless steel mesh.
The “settling tubes” must be connected to the aeration ducts to allow the water from the aeration ducts to flow to the
slotted pipe. The top of the “settling tubes” must be above the treated water level to vent the air between the aeration duct
and the “setting tubes”.
Slotted pipe manifold system results in uniform distribution of water in a small limestone contactor unit.
A 150 mm deep later of 25 mm diameter granite aggregate is required between the limestone bed and the manifold
system. The granite used must be appropriate for water treatment.
At least two drain/flush valves are required for each limestone contactor unit, but four valves are preferred.
The limestone contactor must be designed such that the treated water level is always less than the height of the “settling
tubes” and is 200 mm above the limestone bed. A fully enclosed pipeline must be used to collect and transport the treated
water to the storage tank to prevent contamination.
The pipe must be positioned such that the treated water level is 200 mm above the limestone bed.
Loading rate must be less than 10 m/hr. Higher loading rates will result in increased water turbidity.